1. Field of the Invention
3D sensors for the three-dimensional detection of shape are used in many applications. The measurement of specular surfaces is also possible with optical sensors. Surfaces of spectacle lenses or car windshields are examples thereof.
The optical measurement of specular free-form surfaces takes place through the use of methods which are referred to as “deflectometry.” As is seen in FIG. 1, one or more cameras 8 are directed onto the specular surface of a measurement object 7 and there observe a reflected image of an extended lighting device 1, which is generally a structured screen, a matt sheet illuminated in structured fashion or a television or monitor which represents structures. The specular surface itself is not visible. By evaluating the structures of the reflected image, conclusions can be drawn as to the local inclination of the specular surface and therefore its shape.
Specific mention should be made of raster reflection methods [Ritter83] [Pérard97] [Kammel03]. In those methods, patterns are projected onto a screen.
[Pérard97] also proposes using a television or monitor as the screen. A distortion is usually determined by a series of displaced patterns.
If a sinusoidal pattern is used as the pattern [Häusler 99], a phase shift method can be used to evaluate the images. The required structures on the lighting device are then changeably horizontal or vertical strips. The tracing of a deflected beam 6, which is only available individually in the computation model, at the point of intersection of a vertical line with a horizontal line, takes place in accordance with the laws of reflection. The evaluation takes place in a suitable computation and evaluation unit.
Such an evaluation provides, as a measured variable, a combination of the local surface normal and the local height of the specular surface.
Additional measures are required for an absolute measurement. In [Petz03], the screen is displaced for that purpose. In [Knauer05], two cameras which are used for solving that problem each measure a different combination of height and surface normal.
The optical sensors which function deflectometrically are conventionally calibrated in accordance with photogrammetric methods. In the calibration, the location of the cameras, the inner parameters of the cameras and their lenses, the plane of the screen, and the location of the structures projected onto it are determined during the calibration. When all of those variables are known, the calculation of the profiles of the light beams 6, which is typical for the deflectometry is possible, from the matt sheet to the unknown object and then from the unknown object to the camera.
A measurement system which functions on the basis of that method is already available on the market for measuring aspherical lenses, in particular of spectacles [3DS06]. For that purpose, a screen formed of plastic which is used is illuminated by a video projector. The measurement accuracy for the local inclination of the surface is in the region of 5 minutes of arc.
When measuring specular objects 7 which are also transmissive to visible light, the problem of “rear side reflection” results. Light enters the object 7 and is reflected on optical interfaces positioned further inwards. That reflected light is not desirable; it adds to the light reflected on the surface and falsifies the measurement result. Normally, the rear side of a lens to be measured is that disruptive interface.
In the product [3DS06], that problem is solved by virtue of the fact that                a) the surface of the rear side of the objects to be measured is roughened by being blasted with glass beads,        b) the rear side is painted with a black color having a refractive index which corresponds to that of the object.        
2. Disadvantages of the Prior Art
The methods described have the following problems which are solved in accordance with the invention:                a) Rear side reflection: the blasting of glass beads onto and blackening of lenses is a complex process, in which the lenses are also destroyed. Direct in-line monitoring of the lenses being produced is therefore not possible.        b) Problems when calibrating due to a deviation from the plane: the desired measurement accuracy of the systems in the region of a few minutes of arc presupposes precise calibration methods. The screen is approximated as the plane. The screens used are approximately 50 cm large and are not ideally flat (television screens or monitor displays, extended matt sheets formed of plastic). That is especially the case since they are subject to the influence of the force of gravity because, in general, the measurement object is inserted into the sensor horizontally, and the screen is mounted above it horizontally or inclined. The fact that the real shape of such screens deviates from a plane results in systematic errors in the measurement results. The imaging optical unit of a video projector also generally has distortion, with the result that the projected patterns or lines have slight systematic deformations.        c) Problems in the calibration due to different media: when using a television screen or monitor display as the screen, there is also the problem that a plastic or glass plate is located in front of the (imperfect) plane of image generation. It is not possible with the software which is generally used in photogrammetric calibration to take into consideration the presence of that plate. That likewise results in systematic errors.        d) Problems associated with the stability of the mechanism: televisions or monitors generally have housings formed of plastic. The same applies to video projectors. Fixing through the plastic housing is not mechanically immovably permanent, precisely also over relatively long periods of time, and results in deviations from the calibrated standard and therefore in measurement errors. With the video projector, it is also the case that the optical system is held in the plastic housing and therefore is not fixed reliably in relation to the light modulator.        e) Problems due to temperature influences: when using a television as the screen, it will heat up and expand once it has been switched on. When using a video projector, the image-producing light modulator in the video projector will heat up and expand once it has been switched on, as will the imaging optical unit located in the vicinity thereof. The compensation of those effects is complicated because, in addition, the temperature of the ambient air fluctuates, particularly under production conditions. Video projectors control their temperature in a closed control loop, with the result that the temperature, which is subject to the control process, of the light modulator, firstly fluctuates as a function of time and secondly depends on the temperature of the ambient air. That results in time-dependent and temperature-dependent measurement errors.        